The surface tension of many organic substances is lowered when the substances are dissolved in water. By measuring the surface tension, therefore, it is possible to determine the presence and the quantity of certain organic substances in the water.
One practical application of this method is in controlling the moistening agent in offset printing with the aid of surface-active substances. For use in process engineering the surface tension must be determined using measuring techniques with the aid of a sensor.
In a method developed by the applicants and disclosed in German Offenlegungsschrift No. 32 01 410 a circular region of the surface of the fluid is excited to oscillation so that a system of circular standing capillary waves is produced the occurrence of which is determined by measuring the intensity of a light beam reflected from the surface of the fluid.
The object of the present invention is to develop this method further so that it facilitates a particularly simple, automatic and largely maintenance-free operation and in which the measurement is very insensitive to vibration. The latter characteristic is particularly important for practical applications since the measurement must be made in the immediate proximity of running printing machines.